generated texture displacement adds a line to the displacement
You are presumably using a Blender version greater than 2.79b - eg, 2.79.5 - where the Displacement socket is now a vector rather than a scalar value (blue rather than gray). You need to add a Displacement node before the Displacement socket to convert the ‘height’ from your texture into a vector along the normal of the surface, rather than interpreting it as a cector in world space.